A W-CDMA (Wideband-Code Division Multiple Access) communication scheme has already been developed and come into service as a mobile communication means of third generation. The transmission power control scheme of the W-CDMA communication system includes inner loop control and outer loop control.
In inner loop control, an increase/decrease of the power of a base station from which a portable terminal directly receives a radio wave is controlled in a short period. More specifically, a T-SIR (Target SIR (Signal to Interference Ratio)) is used as a control target value. The T-SIR is compared with a reception SIR measured from a reception signal in the portable terminal. If the reception SIR is lower than the T-SIR, a request to raise the transmission power is issued to the base station. If the reception SIR is higher than the T-SIR, a request to reduce the transmission power is issued to the base station.
In outer loop control, an appropriate T-SIR to be used as a target value of inner loop control is determined on the basis of the long-term average of BLERs (BLock Error Rates). More specifically, a required reception quality (Target BLER (T-BLER)) is obtained from a base station. If a BLER measured in a portable terminal is higher than the T-BLER, the T-SIR is raised. If the measured BLER is lower than the T-BLER, the T-SIR is reduced.
Conventional outer loop control will be described in more detail with reference to FIG. 8. A despreading unit 1101 despreads a reception signal and sends the generated signal to a Rake reception unit 1102. The Rake reception unit 1102 Rake-receives and demodulates the signal and sends it to a long section quality measuring unit 1103. The long section quality measuring unit 1103 calculates the BLER of the demodulated signal for a predetermined period and sends the calculation result, i.e., a measured BLER 1203 to a comparison determination unit 1105.
A T-BLER 1104 holds a T-BLER that is a required reception quality obtained from a signal from the base station. The T-BLER 1104 sends a T-BLER 1204 as a required reception quality to the comparison determination unit 1105. The comparison determination unit 1105 compares the measured BLER 1203 calculated by the long section quality measuring unit 1103 with the T-BLER 1204 and sends a comparison result 1205 to a T-SIR generation unit 1106. The T-SIR generation unit 1106 calculates a T-SIR 1206 from the comparison result 1205 and sends it to the inner loop control side (not shown).
In this example, the long-term average of BLERs is used as a reception quality in outer loop control. Reference 1 (Japanese Patent Laid-Open No. 2003-32184) proposes a technique using a BER (Bit Error Rate) as a reception quality.